1. Field of the Invention
The present invention generally relates to optical recording media, and more particularly to a phase-change type optical recording medium such as a phase-change type optical disk, in which an optical change is caused in a substance of a recording layer by projecting a light beam thereon to thereby record information on the recording layer and reproduce information therefrom and to erase and rewrite information.
2. Description of the Related Art
A phase-change type optical recording medium is well known which utilizes a transition between a crystal phase and an amorphous phase or a transition between a crystal phase and another crystal phase. Such a phase-change type optical recording medium is an information recordable, reproducible and rewritable medium. More particularly, information can be overwritten onto the phase-change type optical recording medium by a single light beam, and the medium can be driven by a simple optical drive system. Since the phase-change type optical recording medium has the above features, it is applied as a recording medium in the computer-related fields and video/acoustic fields. The following are known as the recording substance: GeTe, GeTeSe, GeTeS, GeSeS, GeSeSb, GeAsSe, InTe, SeTe, SeAs, Ge--Te--(Sn, Au, Pd), GeTeSeSb, GeTeSb, Ag--In--Sb--Te.
Japanese Unexamined Patent Publication No. 57-208648 discloses a recording layer buried in a mother substance of SiO.sub.2 or the like in order to suppress a non-reversible change of the recording substance. Particularly, the recording substance of Ag--In--Sb--Te has a feature in which it is highly sensitive and the contour of an amorphous portion is definite, and is thus applied to mark-edge recording (see Japanese Unexamined Patent Publication Nos. 2-37466, 2-171325, 2-415581 and 4-141485).
A multilayer structure including the recording layer made of the above recording substance is known. Such a multilayer structure includes, in addition to the recording layer, a reflection layer, a first protection layer and a second protection layer. Nowadays, it is required for the multilayer recording medium that the repetitive recording performance be improved and the recording performance be compatible with other performances such as the degree of modulation and given reflection rate.
Regarding the above requirement, the following documents show that a flow of the recording layer is suppressed and the repetitive recording performance can be improved by adding nitrogen (N) or the like to the recording layer: Japanese Unexamined Patent Publication Nos. 4-11336, 4-10980, 4-10979, 4-52188 and 4-52189.
An optical recording medium specifically applied to a less-expensive optical recording system having a relatively low recording linear velocity or used as an optical recording medium (CD-RW: Compact Disk ReWritable) that is reproduction-compatible with the CD (Compact Disk) has a limited number of times that information can be repeatedly recorded, normally, hundreds to millions of times. This is because the recording layer is flowed or a film of the multilayer structure is flaked off due to a thermal shock caused at the time of projecting the laser beam thereon or because a metal used for the reflection layer is degraded. Such a limited number of times that information can be repeatedly recorded is not suitable for an application in which rewriting is frequently carried out, for example, peripheral devices of the computer system. Further, it is desired that the recording sensitivity be improved in order to reduce the production cost.